Unstable Mass Transfer from a Main-sequence Star to a Supermassive Black Hole and Quasiperiodic Eruptions

We discuss the formation and evolution of systems composed of a low-mass (M _⋆ ≲ 4 M _⊙) main-sequence star orbiting a 10⁵-10⁷ M _⊙ supermassive black hole with an orbital period of order ∼hours and a mild eccentricity (e ≈ 0.1-0.2), episodically shedding mass at each pericenter passage. We argue that the resulting mass transfer is likely unstable, with Roche lobe overflow initially driven by gravitational-wave emission, but then being accelerated by the star’s expansion in response to its mass loss, undergoing a runaway process. We show that such systems are naturally produced by two-body gravitational encounters within the inner parsec of a galaxy, followed by gravitational-wave circularization and inspiral from initially highly eccentric orbits. We argue that such systems can produce recurring flares similar to the recently identified class of X-ray transients known as quasiperiodic eruptions, observed at the centers of a few distant galaxies.